feat(editor): move preview layout controls into the tab strip (#554)

Markdown layout controls occupied a permanent bordered popup over the document.

- Move them into the editor tab strip with layout icons. When tabs are hidden, including Distraction Free Mode, show one floating toolbar that appears on mouse activity and fades after inactivity.
- Use the same floating toolbar for the HTML browser controls.
- Keep keyboard access: hold the toolbar while focused, return focus on Escape, and respect reduced motion. Add theme tokens with their import, template and reference entries.

src/features/web/Web.module.css

@@ -15,28 +15,12 @@
   border-right: 1px solid var(--border-editor-tabs, var(--border));
 }
 
-/* Below the inspection widget; shown while the pointer is over the editor, like IDEA. */
-.bar {
-  position: absolute;
-  top: 32px;
-  right: 20px;
-  z-index: 5;
+.actions {
   display: flex;
-  gap: 1px;
-  padding: 2px;
-  border: 1px solid var(--border-popup);
-  border-radius: var(--radius-toolbar-button);
-  background: var(--bg-popup);
-  box-shadow: var(--shadow-popup);
-  opacity: 0;
-  transition: opacity 120ms ease;
+  align-items: center;
 }
-.editor:hover .bar {
-  opacity: 0.85;
-}
-.bar:hover,
-.bar:focus-within {
-  opacity: 1;
+.browserBar {
+  top: var(--floating-toolbar-browser-top);
 }
 
 .preview {